Magnetic flux pinning and flux jumps in polycrystalline MgB 2 Kunitoshi Murai a , JunÕya Hori a , Yoshiko Fujii a, * , Jonah Shaver b , Gregory Kozlowski b a Department of Applied Physics, Okayama University of Science, Ridaicho 1-1, Okayama 700-0005, Japan b Wright State University, Dayton, OH 45435, USA Received 1 October 2004; received in revised form 3 March 2005; accepted 11 March 2005 Abstract MgB 2 polycrystalline samples were fabricated under varying conditions of isostatic pressing in argon gas. The critical current densities (J C ) were determined through measurements of hysteresis loops, and the highest value of J C at 10 K was 1.9 · 10 4 A/ cm 2 at 4.8 T. The depinning temperatures were measured at various magnetic fields using the vibrating reed technique. Flux jumps appeared below 7.4 K. The hysteresis loops were carefully examined to determine the temperature and magnetic field range where flux jumps appeared. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: MgB 2 polycrystal; Hysteresis loop; Critical current density; Flux jump 1. Introduction Since the discovery of the new superconductor MgB 2 , many studies have been conducted on its basic proper- ties and practical applications. From a practical point of view, the value of the critical current density, J C , is important. J C for polycrystalline MgB 2 is high at zero field, but falls rapidly with increasing magnetic field compared with conventional metallic superconductors [1–3]. The grain boundaries of MgB 2 powder do not show weak coupling. Thus, high-density bulk samples have been fabricated by high-pressure synthesis to ob- tain samples having a high J C [1,4]. We have also pre- pared samples by high-pressure sintering under varying conditions. The magnetic hysteresis loops were mea- sured and the J C values were obtained. Using the vibrat- ing reed technique, the flux depinning was measured. The appearance of flux jumps reported at low tempera- ture and low field is detrimental in practical applications [5–7]. We have carefully studied the behavior of flux jumps by hysteresis loop measurements below 8 K. 2. Experimental 2.1. Sample preparation Samples were prepared from MgB 2 powder (325 mesh size, 98% purity, Alfa Aesar Inc.). Each pellet (5 g) was uniaxially pre-pressed at 140 MPa for 1 h in a 0.5-in. die, then encased in an evacuated stainless steel (SUS 304) can, and hot isostatically pressed under 207 MPa at elevated temperature for 2 h. This procedure was performed in a furnace under argon gas atmosphere. The evacuation was performed at room temperature (sample Nos. 1 and 2) or at 500 °C (sample Nos. 3, 4, and 5) for outgassing. The sintering temperature was 900 °C (sample No. 3), 950 °C (sample Nos. 1 and 4) or 1000 °C (sample Nos. 2 and 5). The temperature and pressure were increased at a rate of 15 °C/min and 3.45 MPa/min, respectively. After the 2-h sintering run, the temperature was ramped 0011-2275/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2005.03.001 * Corresponding author. Tel./fax: +81 86 256 9470. E-mail address: yoshiko@dap.ous.ac.jp (Y. Fujii). Cryogenics 45 (2005) 415–420 www.elsevier.com/locate/cryogenics